His (Hiranyakasipu's) decision was that since the self (atma) and time are both eternal, if he could not occupy that post (of Lord Brahma) in one lifetime he would continue to execute austerities life after life so that sometime he would be able to do so

Expressions researched:
"his decision was that since the self (ātmā) and time are both eternal, if he could not occupy that post in one lifetime he would continue to execute austerities life after life so that sometime he would be able to do so"

Srimad-Bhagavatam

SB Canto 7

The duration of Brahmā's life is extremely great, and consequently it was impossible for Hiraṇyakaśipu to occupy that post. Nonetheless, his decision was that since the self (ātmā) and time are both eternal, if he could not occupy that post in one lifetime he would continue to execute austerities life after life so that sometime he would be able to do so.

SB 7.3.9-10, Translation and Purport

"The supreme person within this universe, Lord Brahmā, has gotten his exalted post by dint of severe austerities, mystic power and trance. Consequently, after creating the universe, he has become the most worshipable demigod within it. Since I am eternal and time is eternal, I shall endeavor for such austerity, mystic power and trance for many, many births, and thus I shall occupy the same post occupied by Lord Brahmā.

Hiraṇyakaśipu's determination was to occupy the post of Lord Brahmā, but this was impossible because Brahmā has a long duration of life. As confirmed in Bhagavad-gītā (BG 8.17), sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ: one thousand yugas equals one day of Brahmā. The duration of Brahmā's life is extremely great, and consequently it was impossible for Hiraṇyakaśipu to occupy that post. Nonetheless, his decision was that since the self (ātmā) and time are both eternal, if he could not occupy that post in one lifetime he would continue to execute austerities life after life so that sometime he would be able to do so.